This invention relates to a pulse arc discharge welding apparatus, and more particularly to a pulse arc discharge welding apparatus in which, with a discharge current waveform provided in the form of high frequency pulses, the discharge light and the gas in pulse discharge and the electrode are regulated and controlled with high accuracy.
The pulsed arc discharge apparatus can prevent variations in the depth of penetration and in the area of section of a base metal which contributes for instance to the variation in height of the welding torch.
An example of the pulse discharge operated apparatus of this type is a pulse arc discharge welding apparatus where a pulse arc current flows between a consumable welding wire electrode (hereinafter referred to merely as "a wire electrode", when applicable) and an object to be welded, to generate heat through pulse arc discharge, so that the heat melts the object and a filter metal, while the melted filter metal is moved to the welding part of the object by the electromagnetic pinch force of pulse arc discharge, thus accomplishing the welding.FIG. 1 shows a conventional pulsed arc discharge apparatus disclosed, for instance, by Japanese Patent Application (OPI) No 19184/1982 (the term "OPI" as used herein means an "unexamined published application"). In FIG. 1, reference numeral 1001 designates a pulsed arc welding power source; 1002, consumable electrode wire (hereinafter referred to as "a welding wire" when applicable); 1003, a welding torch; 1004, discharge arcs; 1005, a base metal; 1006, a voltage detector for detecting a voltage V.sub.F developed across the welding torch 1003 and the base metal 1005; 1007, a comparator for comparing the voltage V.sub.F detected with a predetermined welding voltage V.sub.o, to output the difference voltage Of (V.sub.o -V.sub.F); 1008, welding a voltage setting unit for setting the above-described welding voltage V.sub.o ; 1009, a pulse peak value setting unit for setting a pulse peak value I.sub.P ; 1010, a base current setting unit for setting a base current I.sub.B ; and 1011, a pulse width setting unit for setting a pulse width .tau.. Further in FIG. 1, reference character Ex designates the length of extension of the welding wire 1002, .lambda.a, an arc length; and .lambda.x, the distance between the welding torch 1003 and the base metal 1005. The pulse arc welding power source 1001 includes a pulse waveform shaping section, which is not shown for simplification in description.
The construction of the conventional pulsed arc discharge apparatus is as described above. With the pulsed arc welding power source, the current obtained by superposing a pulse current shown in FIG. 2A or FIG. 2B (the pulse peak value I.sub.P, and the pulse width .tau.) On the DC base current I.sub.B is applied between the welding wire 1002 and the base metal 1005, so that pulsed arc discharge 1004 is caused to occur between the welding wire 1002 and the base metal 1005; that is, a welding operation is carried out. The welding wire 1002 is fed continuously towards the base metal 1005. The welding wire 1002 thus fed is melted by the pulsed arc discharge 1004, thus being transferred to the base metal 1005. In general, the speed of feeding the welding wire 1002 is proportional to the pulse period.
When the height of the welding torch 1003 changes or the base metal 1005 bends, the distance .lambda.x between the welding torch 1003 and the base metal 1005 is changed, and accordingly the arc length .lambda.a is changed. In order to overcome this difficulty, in the conventional pulsed arc discharge apparatus, the following feedback control is employed:
The voltage detector 1006 detects the voltage V.sub.F at all times. The comparator 1007 compares the voltage V.sub.F thus detected with the welding voltage V.sub.o set by the welding voltage setting unit 1008, to output the difference voltage (V.sub.O -V.sub.F) signal. The difference voltage signal iS applied to the pulse Width setting unit 1011. In the pulse width setting unit 1011, the control pulse data .DELTA..tau. is made to be a function of the difference voltage (V.sub.O -V.sub.F), and the pulse width .tau. is processed so as to be (.tau..sub.0 .+-..DELTA..tau.). Let us consider the case where the shield gas is CO.sub.2 gas, the welding wire feeding speed Vw is 7 m/min, the pulse peak value I.sub.P is 500A, and the base current I.sub.B is 60A. If, when the distance .lambda.x between the welding torch 1003 and the base metal 1005 is changed and the length of extension Ex is accordingly changed from 15.8 mm to 20.4 mm as shown in FIG. 3, the pulse width .tau. is changed from 4.0 ms to 3.6 ms, then the resultant arc length .lambda.a is substantially equal to that in the case where the length of extension Ex is 15.8 mm.
This principle will be described with reference to FIG. 4 in detail.
FIG. 4 is an enlarged diagram showing the welding section. In FIG. 4, reference character V.sub.F designates the voltage developed across the welding torch 1003 and the base metal 1005; Vw, the speed at which the welding wire 1002 is continuously fed; Ex, the length of extension of the welding wire 1002 between the electric feeder of the welding torch and the discharge arc; Q.sub.J, the quantity of heat with which the welding wire 1002 is Joule-heated while moving from the electric feeder of the welding torch to the arc; I, the effective value ##EQU1## of the pulse current; R, the resistance per unitary length of the welding wire 1002; Q.sub.1, the quantity of heat applied to the welding wire 1002 by arc discharge 1004; k.sub.1, the proportional constant determined from the diameter and material of the welding wire; I, the average current (I=(1/T)[I.sub.P .multidot..tau.+I.sub.B (T-.tau.)]; Q.sub.2, the quantity of heat applied to the base metal by arc diSCharge 1004; and k.sub.2, the proportional constant determined from the gas, and the configuration and material of the base metal.
As shown in FIG. 4, the quantity of heat Q.sub.w applied to the welding wire 1002, the quantity of heat Q.sub.J, and the quantity of heat Q.sub.1 can be calculated accOrding to the following expressions, respectively: EQU Q.sub.w =Q.sub.J +Q.sub.1 EQU Q.sub.J =I.sup.2 .multidot.R.multidot.Ex/Vw EQU Q.sub.1 =k.sub.1 I
The amount of melting of the welding wire 1002, being proportional to the quantity of heat Q.sub.w applied to the welding wire 1002, forms a molten drop at the end of the welding wire 1002. The molten drop is transferred to the base metal 1005 by the pulse current.
Thus, the arc length .lambda.a is determined where the welding wire feeding speed Vw is made in balance with the number of molten drops transferred to the base metal 1005. In other words, in order to maintain the arc length .lambda.a constant, it is necessary to maintain the quantity of heat Q.sub.w constant in the following equation (1): EQU Q.sub.w =Q.sub.J +Q.sub.1 =(I.sup.2 .multidot.R.multidot.Ex/Vw)+k.sub.1 I (1)
When, as was described with reference to FIG. 3, a welding operation is carried out with Ex=15.8 mm, the arc length .lambda.a can be held at the most suitable value with a pulse width .tau. of 4.0 ms, and in this case the effective current I is 263A, and the average current I is 177A. In the case where the length of extension Ex is changed from 15.8 mm to 20.4 mm, the arc control on pulse width .tau. is effected, so that the pulse width .tau. is decreased to 3.6 ms so that the arc length .lambda.a is maintained at the value provided when Ex=15.8 mm. As a result, the effective current I becomes 250A while the average current I is 166A, and the quantity of heat Q.sub.w is maintained unchanged.
In the conventional pulsed arc discharge apparatus as described above, in order to prevent the variation of the arc length .lambda.a caused by the variation in the length of extension Ex of the welding wire 1002, the arc length control on pulse width .tau. is carried out. In this case, not only the effective current I but also the average current I is changed. When the average current I changes, the quantity of heat Q.sub.2 applied to the base material 1005 by arc discharge 1004 is changed. As a result, as shown in FIGS. 5A and 5B, the weld penetration is changed in sectional area. If the sectional area of the weld penetration is small, then the welding strength is reduced correspondingly.
The pulse arc discharge welding apparatus will be described in more detail. FIG. 6 is an explanatory diagram showing the arrangement of a conventional pulse arc discharge welding apparatus which has been disclosed, for instance, in Published Unexamined Japanese Patent Application (OPI) No. 19177/1982.
In FIG. 6, reference numeral 1 designates a DC power source; 2, a switching element which comprises a power transistor element which turns on and off the output current of the DC power source 1, thus forming a pulse-shaped current waveform; that is, performing the chopper control of the current; and 3; an arc load section. The arc load section comprises: a welding torch 31, and a wire electrode 32 which is a filter metal in the form of a wire, and is supplied from a wire reel. Further in FIG. 6, reference numeral 4 designates an arc maintaining power source for supplying a continuous base current to the switching element 2 in order to prevent the difficulty that the discontinuation of arc occurs between the above-described pulses; 5, a control circuit for controlling the switching element 2, to set the pulse frequency and pulse width of the pulse current to predetermined values; and 6, a current detector for detecting the current i.
The operation of the pulse arc discharge welding apparatus thus organized will be described.
In general, in the pulse arc discharge welding apparatus, even in the case where the average current is smaller than that in a DC arc discharge welding apparatus, the end portion of the wire electrode 32 is melted with the pulse arc current, and the melted end portion, is disconnected from the wire electrode by the electromagnetic pinch force of pulse arc current, thus forming a molten drop, and the molten drops thus formed are intermittently transferred to the material 34 to be welded (hereinafter referred to as "a welding material 34", when applicable), thus achieving the welding. The transfer of the molten drop will be referred to as "a spray transfer", when applicable. Thus, the pulse arc discharge welding apparatus can perform a welding operation with an average current lower than that in the DC arc discharge welding apparatus. Therefore, the pulse arc discharge welding apparatus is advantageous in that a thinner spray transfer welding of a welding material can be achieved, and the spray transfer eliminates the "spattering" which otherwise may be caused during the welding operation.
The waveform of the pulse arc current employed in the pulse arc welding operation will be described. It has the following functions or characteristics:
FIGS. 7A and 7B show examples of the pulse arc current waveform. In this case, the material of the wire electrode is soft steel, the wire electrode is 1.2 mm.PHI., and the atmospheric gas is a mixture of argon gas and 20% CO.sub.2 gas.
The pulse arc current waveform is so determined that, in the case of a welding operation with a high average current, its pulse period C.sub.B1 is short as shown in FIG. 7A, and in the case of a welding operation with a low average current, its pulse period C.sub.B2 is long as shown in FIG. 7B.
FIG. 8 is a characteristic diagram indicating a relationship between pulse peak current values I.sub.P and molten droplet length .lambda. which can leave the wire electrode having a diameter of 1.2 mm. In FIG. 8, the characteristic curve 16 shows pulse peak current values with molten droplet lengths .lambda. with which such a molten droplet can leave the wire electrode in the case where the atmospheric gas is a mixture of argon gas and 20% CO.sub.2 gas, and the characteristic curve 17 indicates pulse peak current values with such molten droplet lengths .lambda.,in the case where the atmospheric gas is 100% CO.sub.2 gas. When the arc length is above the one-dot chain line 20 in FIG. 8, then welding defects called "undercuts".pi.will be formed; that is, the one-dot chain line 20 is a threshold line. In the shaded region A of FIG. 8, a welding operation can be carried out without formation of undercuts or spatters. That is, the welding operation is satisfactorily achieved through the spray transfer of molten droplets.
In order to perform a pulse arc welding operation satisfactorily, it is essential to prevent the spattering of molten material during the welding operation to prevent the formation of undercuts which are defects formed in welding beads, and to make substantially unifar the size of molten droplets leaving the wire electrode. More specifically in order to prevent the spattering mentioned above, it is necessary to prevent the contact of the wire electrode and the object to be welded; and in order to prevent the formation of undercuts, it is essential to make the arc length short. In order to meet the two requirements at the same time, the pulverization (spray transfer) of the molten drop leaving the wire electrode is required. In order to make uniform the size of molten droplets leaving the wire electrode, a pulse arc current constant in waveform should be applied periodically as shown in FIGS. 7A and 7B.
In the atmospheric gas which is the mixture of argon gas and 20% CO.sub.2 gas, the size of the molten drop which can leave the wire electrode depends on the pulse arc current peak value I.sub.P as indicated by the characteristic curve 16 in FIG. 8.
As is apparent from the characteristic curve 16 in FIG. 8, the length .lambda. of the molten droplet leaving the wire electrode is reduced abruptly when the pulse peak current value I.sub.P is about 300 A, being 0.3 to 0.4 mm when it is 400 A. That is, even when the distance (arc length) between the wire electrode and the welding material is made as short as about 0.4 mm, then the molten metal can be left from the wire electrode with the pulse current; that is, the welding operation can be achieved satisfactorily.
In FIG. 8, the one-dot chain line is obtained through experiment, indicating the maximum arc length with which no undercuts are formed in the welding bead; that is, the welding bead formed is satisfactory in quality. Thus, it can be understood that, in order to prevent the formation of spatters during the welding operation and to obtain high quality welding beads with no undercuts, the pulse peak current value I.sub.P should be at least 300 A.
When, in the case where a pulse current waveform as shown in FIGS. 7A and 7B is employed and the atmospheric gas is a 100% CO.sub.2 gas, the pulse peak current value I.sub.P is 450 A, the molten droplet will not leave the wire electrode having a diameter of 12 mm; and when the pulse peak current value I.sub.P is 500 A, the molten droplet will not leave the wire electrode, having a diameter of 8 mm. As a result undercuts occur in the welding bead. Accordingly, it is estimated that, when the molten droplet is allowed to leave the wire electrode in the atmospheric gas of 100% CO.sub.2, the pulse peak current I.sub.P characteristic is such as indicated by the characteristic curve 17 in FIG. 8.
Phenomena as shown in FIGS. 9A and 9B have been found through observation of the leaving of molten droplets from the wire electrode with the pulse peak, current value I.sub.P set to 500 A. That is, when, as shown in FIG. 9A, the base current I.sub.B is high and the pulse width .tau. is short, the molten droplet 36 formed at the end of the wire electrode changes from P.sub.O to P.sub.a1 in configuration, and finally to P.sub.a2 ; that is, it cannot leave the wire electrode until it becomes bulky. On the other hand, when, as shown in FIG. 9B, the base current I.sub.B is low and the pulse width .tau. is long, the electromagnetic force F of the pulse current acts upwardly, and therefore the molten droplet 36 changes from P.sub.o to P.sub.b2, in configuration, thus being raised, and then to P.sub.b2, thus leaving the wire electrode. The molten droplet thus left may not be dropped onto the welding material; that is, it may be scattered as spatters being rotated at high speed, or it may stick on the wire electrode again as indicated at P.sub.b2', thus forming a larger mOlten droplet which falls onto the welding material.
As is apparent from the characteristic curve 17 in FIG. 8 and FIGS. 9A and 9B, when, in the case where the atmospheric gas is a 100% CO.sub.2 gas, the pulse peak current value I.sub.P is of the order of 500 A, the arc discharge is insufficient in spread, and therefore it is impossible to make smaller the molten droplet transferring onto the welding material.
It can be considered from the characteristic curve 17 in FIG. 8 that, in the case of the 100% CO.sub.2 atmospheric gas, the pulse peak current value I.sub.P should be at least 700 A in order to reduce the size of the molten droplet leaving the wire electrode and to obtain a welding bead fine in quality.
FIG. 10A shows a pulse current waveform, and FIG. 10B schematically shows the transfer of a molten droplet and the change of a molten pool at the time instants (A) through (H) of the waveform. As is seen from FIGS. 10A and 10B, the molten pool is oscillated by the pulse, thus contacting the wire electrode so that sometimes spatters are formed.
FIG. 11A shows a pulse current waveform also, and FIG. 11B schematically shows the transfer of a molten droplet and the change of a molten pool at the time instants A through E of the waveform. FIG. 11B shows the case where, when the droplet transfer, being delayed, occurs during the base period (the point D), the current in the base period is low, and therefore the arc is discontinued at the transfer of the droplet.
The function or performance of the conventional pulse arc discharge welding apparatus is as described above. Therefore, if, in a pulse arc welding operation with a 100% CO.sub.2 atmospheric gas, the pulse peak current pulse I.sub.P set to a small value, then a droplet leaving the wire electrode unavoidably becomes bulky; that is, the droplet formed at the end of the wire electrode may contact the welding material. As a result, it may be scattered as spatters around the welding apparatus, or form welding bead defects, namely, undercuts. Furthermore, the conventional welding apparatus is disadvantageous in that, when the pulse peak current value I.sub.P is set to a high value, then the power source becomes bulky, and accordingly the apparatus is increased in weight as much, and increased greatly in manufacturing cost. In addition the conventional welding apparatus suffers from the following difficulties: As shown in FIG. 10B, the molten pool is vibrated by the pulse, thus contacting the wire electrode, as a result of which spatters are formed. As shown in FIG. 11B, the arc is discontinued at the transfer of a droplet from the wire electrode.